High temperature, solid oxide electrolyte fuel cell generators, which are made of mostly ceramic components, and which allow controlled leakage among plural chambers in a sealed housing, are well known in the art, and taught by Isenberg, in U.S. Pat. No. 4,395,468. Referring to the Drawings, one type of such prior art design is shown in FIGS. 1 and 2, where exterior, gas-tight housing 12 sealingly surrounds three chambers which communicate among one another through controlled gas seepage within the fuel cell generator 10.
The housing 12 surrounds a fuel inlet or generating chamber 14, a combustion product or preheating chamber 16, and an oxidant inlet chamber 18. The housing is lined throughout with a thermal insulation 22, such as low density alumina. Penetrating the housing is a fuel inlet port 24, an air inlet port 25, within oxidant inlet chamber 18, and combustion product outlet port 28 leading hot exhaust gas 39 from combustion product chamber 16. The generator 10 is usually operated in a vertical position as shown.
The fuel cells 40 include a solid oxide electrolyte sandwiched between two ceramic electrodes supported on a porous ceramic support. Of significance in this design is the fact that the porous ceramic barriers 32 and 62 and the metal tube sheet 34, are not sealed structures. Smooth, round, ceramic, oxidant air conduits 20 are loosely supported at one end in the ceramic barrier 62 and tube sheet 34. As shown in FIG. 2, the tube sheet 34 has associated bores 60 that fit loosely about the conduits 20 to allow free thermal expansion. The conduits 20 are comprised of alumina, and the tube sheet is covered with an insulation 62 such as low density alumina. Leakage of oxidant, into the preheating chamber 16, as indicated by arrow 63 in FIG. 2, was considered acceptable, even though during actual operating conditions leakage of oxidant air constituted about 4 vol. % of the air feed 26. The conduits 20 extend from the tube sheet 34 into the open end 42 of the fuel cells 40.
It has been found that leakage of the feed oxidant air into the preheating chamber 16 can be detrimental to fuel cell generator performance. Such leakage lowers the temperature of combustion gas exhausted through outlet port 28 which would ordinarily be used in a heat recoupment device, lowers the preheating capability of generator chamber 14 and degrades the overall efficiency of the fuel cell generator system by about 4%. Additionally, it would be desirable to enhance the heat transfer to the feed air in the preheater-combustion product chamber 16, and in the generating or fuel cell region 14, and to provide a better scheme for discharge of combustion products. Improved heat transfer in these regions could result in a more compact preheater, and a more uniform operating temperature in the fuel cell region.